The human immune system protects the body against bacteria and viruses that cause disease. The system is composed of two parts. The innate immune system includes nonspecific defenses that work against a wide range of infectious agents. This system includes both physical barriers that keep out foreign particles and organisms along with specific cells that attack invaders that move past barriers. The second part of the immune system is the adaptive immune system, which “learns” to respond only to specific invaders.
Lines of Defense in the Immune System
The first line of defense in the immune system are barriers to entry. The most prominent is the skin, which leaves few openings for an infection-causing agent to enter. Bodily orifices exhibit other methods for preventing infection. The mouth is saturated with native bacteria that dominate the resources in the microenvironment, making it inhospitable to invading bacteria. In addition, enzymes in the mouth create a hostile environment for foreign organisms. The urethra flushes away potentially invasive microorganisms mechanically through the outflow of urine, while the vagina maintains a consistently low pH, deterring potential infections. The eyes and nose constantly produce and flush away tears and mucus, which trap pathogens before they can replicate and infect.
Pathogens do occasionally breach these barriers and arrive within the body, where they attempt to replicate and cause an infection. When this occurs, the body mounts a number of nonspecific responses. The body’s initial response is inflammation: infected cells release signaling molecules indicating that an infection has occurred, which causes increased blood flow to the area. This increase in blood flow includes the increased presence of white blood cells, also called leukocytes. The most common type of leukocyte found at sites of inflammation are neutrophils, which engulf and destroy invaders
Phagocytosis occurs when a cell completely surrounds a particle to form an enclosed vesicle. The particle can then be broken down either for nutrients or to neutralize a threat. Cells in the immune system that use phagocytosis are called macrophages.
Other innate responses include antimicrobial peptides, which destroy bacteria by interfering with the functions of their membranes or DNA, and natural killer lymphocytes, which respond to virus-infected cells. Because they can recognize damaged cells with the presence of antibodies, they are important in early defense against bacterial infection. In addition, infected cells may release interferon, which causes nearby cells to increase their defenses.
Types of White Blood Cells
Type of Cell
Name of Cell
Innate or Adaptive
First responders that quickly migrate to the site of infections to destroy bacterial invaders
Attack multicellular parasites
Large cell responsible for inflammatory reactions, including allergies
Respond to antigens by releasing antibodies
Respond to antigens by destroying invaders and infected cells
Natural killer cells
Destroy virus-infected cells and tumor cells
Innate and adaptive
Engulf and destroy microbes, foreign substances, and cancer cells
Innate and adaptive
The adaptive immune system is able to recognize molecules called antigens on the surface of pathogens to which the system has previously been exposed. Antigens are displayed on the surface of cells by the major histocompatibility complex (MHC), which can display either “self” proteins from their own cells or proteins from pathogens. In an antigen-presenting cell, the MHC on the cell’s surface displays a particular antigen, which is recognized by helper T-cells. These cells produce a signal (cytokines) that activates cytotoxic T-cells, which then destroy any cell that displays the antigen.
The presence of antigens also activates B-cells, which rapidly multiply to create plasma cells, which in turn release antibodies. Antibodies will bind only to specific antigens, and in turn result in the destruction of the infected cell. Some interfere directly with the function of the cell, while others draw the attention of macrophages. Memory B-cells are created during infection. These cells “remember” the antigen that their parent cells responded to, allowing them to respond more quickly if the infection appears again.
Together, T- and B-cells are known as lymphocytes. T-cells are produced in the thymus, while B-cells mature in bone marrow. These cells circulate through the lymphatic system.
Infections can be caused by many different infectious agents, including bacteria, viruses, protozoa, fungi, and parasites. Some infections will attack the immune system itself. Human immunodeficiency virus (HIV) attacks helper T-cells, eventually causing acquired immunodeficiency syndrome (AIDS), which allows opportunistic infections to overrun the body.
The immune system itself can also be pathological. The immune system of individuals with an autoimmune disease will attack healthy tissues. Autoimmune diseases (and the tissues they attack) include: